Several laser types emit their energy in the form of short pulses. For certain applications, it is necessary to utilize this energy with an extended pulse length (and correspondingly reduced peak power).
This is usually true for high energy pulses. With these lasers, the energy distribution within the beam cross section can change from pulse to pulse, and it is usually necessary to homogenize the energy distribution within the beam cross section.
Several approaches have been considered in order to change the time/power relationship of the pulse. One was to change the modulation of the laser itself, but this is sometimes limited by the discharge physics and thus an optical method is needed.
The traditional approach to extend the pulse width is to use a combination of partial reflectors to divide the energy and then reflect the various portions, after they have traveled a certain distance, thus accumulating the necessary delays. Let's consider, as an example of this method, the case of extending the pulse width by 3. The first partial reflector will have a reflection coefficient of close to 33%, thus, 67% of the energy will pass on until they reach, after a delay corresponding to a full pulse width, the next partial reflector with a reflection coefficient of 50%.
50% of this energy, that is, about 33% of the total energy, will be reflected back (to follow the first 33% reflected by the first reflector).
The last 33% of the energy will pass through the second reflector, and after a further delay equal to a pulse width, will reach a 100% reflection to follow the first two portions.
These two last portions, on their way back, will have to pass through the partial reflectors and each will lose a portion of the energy to secondary division (the 33% reflected by the second reflector will have to pass through the 33% first reflector, but only 67% of this energy will pass on). This secondary reflection will appear as a "tail" of gradually diminishing intensity.
The advantage of this system is that by using flat partial reflectors, it is not sensitive to energy distribution variations.
The long delays also require that some compensation means for the beam divergence be incorporated (such as using concave reflectors to effect the delay), and thus make the delay an imaging rather than a plain transmission scheme.
The inherent disadvantage of this system is the percentage of energy transferred to the "tail", due to secondary reflections. Usually, this energy cannot be utilized and represents a significant loss.